The present invention relates to an electrohydraulic control for an automatic transmission of a motor vehicle having a selector lever, with selected gear positions P, N, D, 3, 2, and 1, transmitted to an electric control unit and which is mechanically connected with a hydraulic shift slide with which several shift valves are connected which are used for the hydraulic control of the automatic transmission. The control unit controls solenoid valves which shift the individual transmission gears together with one-way clutches as well as clutches and brakes which can be acted upon by pressure.
From the German Publication DE-Z ATZ 85 (1983) 6, Pages 393 to 398 and 401 to 405, a 4-speed automatic transmission is known from two publications which comprises a Simpson-type system with a rear-mounted overdrive and an electrohydraulic control. A wiring diagram for the electrohydraulic control is illustrated in Publication 4 HP 22 E of the ZF Getriebe GmbH Saarbrucken. By means of this control, in the driving position D and the shifted first gear, the engine torque is transmitted to the transmission output shaft by way of a one-way clutch. In the coasting operation, the one-way clutch is overridden so that the vehicle rolls freely and the braking torque of the engine cannot be transmitted.
For this control and other previously known controls, it is customary to carry out the downshifting into the first gear in the driving position D at approximately 10 to 30 km/h. The hydrodynamic converter of the automatic transmission makes it possible in the normal driving operation to start in the second gear so that the first gear must be used only in extreme situations.
However, a new electrohydraulic transmission control makes it possible, for power shift transmissions, to manually provide a certain gear so that it would now be possible in the driving position D, to engage the first gear below approximately 75 km/h. But the brake boosting by the engine, which is absent in contrast to the manual shift gear, would force the driver to use the service brake in order to adapt his speed to the existing traffic situation. For this reason, such a shifting operation from the second into the first gear is currently not possible by means of an automatic transmission of a conventional construction.
The German Patent Document DE-39 34 674 A1 shows an electrohydraulic control device for an automatic transmission in which a clutch which, when the first gear is engaged, establishes the power flux in the coasting operation. This clutch is not connected mechanically by means of the selector lever but is temporarily connected by the control unit by way of electromagnetic valves. Conditions for the connecting of the clutch are not disclosed in that document.
An object of the invention is to provide an electrohydraulic control for this type of an automatic transmission in which in the D position of the selecting slide, in the first gear, a transmitting of the coasting torque to the engine becomes possible by way of the transmission. In other words, a coasting connection of the engine without any significant additional costs.
This and other objects are achieved by the present invention which provides an arrangement for the electrohydraulic control of an automatic transmission of a motor vehicle having a selector lever, with gear positions P, N, D, 3, 2 and 1 being selected via the selector lever 2. The arrangement comprises solenoid valves which shift individual transmission gears together with one-way clutches as well as clutches and brakes which can be acted upon by pressure. An electric control unit is coupled to receive the selected gear positions from the selector lever, the control unit controlling the solenoid valves. A hydraulic shift slide is mechanically connected to the selector lever and a plurality of shift valves are connected thereto, this plurality of shift valves hydraulically controlling the automatic transmission. A first one of the one-way clutches is assigned to the first gear, and a first one of the brakes is parallel to the first one-way clutch. For the shifted gear position D and a shifted first gear, the first one-way clutch is bridged by admission of pressure to the first brake.
An advantageous feature of the present invention is that the braking effect of the engine can also be utilized in the first gear in the driving position D. For this purpose, only a few simple constructive measures are required which can be carried out without significant additional expenditures. The object of the invention can also be easily implemented for existing transmissions.
In addition, in the present invention the braking effect of the engine in the first gear exists at any point in time, and the action of the vehicle can therefore be predicted by the driver.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.